Tutorial

Glacial / Periglacial

Glacial and periglacial features are found on both Earth and Mars. A glacial feature is a landform related to the presence and movement of very large ice bodies (i.e. glaciers). On Earth, they are typically found on the tops of tall mountains and in very cold climates such as Greenland or Antarctica. On Mars, however, they are typically found in the middle and high latitudes (roughly 30°–90° latitude in either hemisphere).

Permafrost is a term used to describe ground that is frozen for more than two years. On Earth, permafrost regions occur in the Arctic and Antarctic. When permafrost eventually thaws, characteristic landforms are created. These characteristic landforms are called periglacial features. Examples of common periglacial features include pingos and polygons.

Lobate Debris Aprons

Lobate debris aprons are formed when ice flows down the slope of a cliff or mound. They are glacial features and tend to appear round and flat in satellite imagery. Ground penetrating radar data has revealed that these aprons still contain ice today beneath a thin layer of protective dirt!

Practice identifying lobate ebris aprons

Tongue in Martian fretted terrain
Lobate debris apron coming off a small massif
Channels with fans

These are glacial tongues.

This pair of shield volcanoes on Mars is Uranius Tholus (top) and Ceraunius Tholus (bottom). While they are volcanic features, they are much larger than cinder cones or rootless cones. The term "tholus" is used for smaller mountains or volcanoes on Mars, while large volcanoes bear the name “mons,” such as Olympus Mons, the largest volcano in the solar system.

These are lobate debris aprons.

The arrows in this image point to a lobate debris apron flowing off a small massif (mountain) near the large Hellas basin on Mars.

These are channels with fans.

These features are small channels cut into a crater wall with fans of sediment at their bases.

Concentric Crater Fill

Potential glacial features can occur inside the sheltered bowl of a crater. Sometimes the edges of these features make concentric rings, like the inside of a tree. This feature is called concentric crater fill and is thought to have contained ice in the past, and may still contain ice today beneath a protective layer of dust and regolith (Martian soil).

Practice identifying concentric crater fill

Mid-latitude crater fill in a southern hemisphere crater
Simple crater in the Libya Montes
Concentric crater fill in a northern mid-latitude crater

This is mid-latitude crater fill.

The material on the floor of this crater doesn’t show concentric (ring-like) features, so it is not concentric crater fill. It shows lines suggesting the material flowed toward the south/southeastern portion of the crater.

This is a simple impact crater with no fill.

This bowl-shaped crater in this image does not have concentric crater fill on its floor.

This is concentric crater fill.

This crater has been filled with material displaying concentric (ring-like) lines.

Lineated Valley Fill

Lineated valley fill (LVF) occurs within valleys on Mars. It has parallel lines in the middle of it that look similar to glacial flow lines on glaciers on Earth, leading scientists to believe it is a glacial feature on Mars.

Practice identifying lineated valley fill

Mid-latitude crater fill in a southern hemisphere crater
Simple crater in the Libya Montes
Concentric crater fill in a northern mid-latitude crater

This is lineated valley fill.

This Martian valley has been filled with this potentially icy material. The lines on it suggest the material flowed through this valley.

This valley doesn’t contain lineated valley fill.

There is no material within this valley displaying flow lines, so it does not have any lineated valley fill on its floor.

These are streamlined islands.

While the shape of these islands shows that water flowed around obstacles to carve them, there is no lineated valley fill within this channel.

Mid-Latitude Crater Fill

Possible glacial features in the mid-latitudes that occur inside the bowls of craters are called mid-latitude crater fill. This material tends to form domes on the floors of craters, and can be smooth, show lines suggesting it flowed in a particular direction, and/or have a rough-looking surface. Mid-latitude crater fill is thought to have contained ice in the past, and may still contain ice today beneath a protective layer of dust and regolith.

Practice identifying mid-latitude crater fill

Mid-latitude crater fill in a southern hemisphere crater
Simple crater in the Libya Montes
Concentric crater fill in a northern mid-latitude crater

This is mid-latitude crater fill.

These are a simple impact craters with no fill.

These bowl-shaped craters do not have any mid-latitude fill on their floors.

This is concentric crater fill.

This crater has been filled with material displaying concentric (ring-like) lines, a defining characteristic of concentric crater fill rather than mid-latitude crater fill.

Pingos

Pingos are circular mounds found in periglacial environments. They form in areas of wet ground where permafrost conditions lead to freezing and the formation of ice wedges, which push the ground upwards into a mound as the ice expands. Small pingos tend to have rounded tops, while large pingos have crater-like depressions at their peaks where the ice wedge has begun to melt and break. If the ice wedge melts entirely, the pingo will collapse, leaving behind a crater-like feature.

Practice identifying pingos

Mid-latitude crater fill in a southern hemisphere crater
Simple crater in the Libya Montes
Concentric crater fill in a northern mid-latitude crater

This is a shield volcano.

Look at the scale bar in this image—this feature is very large! Pingos only tend to be tens to hundreds of metres in diameter, while this over 100 km in diameter. The broad shield shape and caldera at the centre tells us this is a shield volcano.

These are volcanic cones.

This is tricky, because pingos and volcanic cones look so similar in satellite images that even scientists can't always tell them apart without geologic context! They might look like impact craters at first glance, but the downward slopes beyond the rims of the "craters" show that these features are elevated and cone-shaped. These particular cones appear to have old lava flows surrounding them, suggesting that these are cinder cones (formed by erupted volcanic material piling up around a vent) as opposed to rootless cones (formed by hot lava moving across water or wet ground).

Yes, these are pingos!

This is tricky, because pingos and volcanic cones look so similar in satellite images that even scientists can’t always tell them apart without geologic context! They might look like impact craters at first glance, but the downward slopes beyond the rims of the “craters” show that these features are elevated and cone-shaped.

Polygons

Polygons (also called "polygonal terrain" or "patterned ground") are shapes in the ground outlined by cracks resulting from repeated freezing and thawing of the ground. They are periglacial features that form in permafrost regions.

Practice identifying polygons

Mid-latitude crater fill in a southern hemisphere crater
Simple crater in the Libya Montes
Concentric crater fill in a northern mid-latitude crater

Yes, these are polygons!

These polygons are located in the permafrost terrain of a crater floor in the northern high latitudes of Mars.

These are south polar pits.

These features are pits with a rounded appearance, as opposed to angular polygons with multiple sides. The pits are in the south polar cap of Mars, which is composed of carbon dioxide ice.

These are sand dunes.

While they do have a roughly polygonal shape to them, these are actually sand dunes covered in frost near the northern polar cap of Mars. The dark areas are places where the dunes have begun to defrost, exposing the dark material of the sand dune hidden beneath the white frost.

Tongues

Tongues are a specific kind feature on Mars, thought by scientists to possibly be dust-covered glaciers (or remnants of glaciers). As the name suggests, their shape resembles the rounded lobe of a tongue. They are often found on confined slopes of crater or valley walls.

Practice identifying tongues

Mid-latitude crater fill in a southern hemisphere crater
Simple crater in the Libya Montes
Concentric crater fill in a northern mid-latitude crater

This is a landslide.

This landslide may somewhat resemble the shape of a tongue, it is covered with boulders (the dark spots) and lacks flow lines seen in glacial tongues.

This is a glacial tongue.

Notice the shape of the feature and the lines suggesting that it has flowed downhill. The ridges around the edges of the tongue resemble glacial moraines—debris that has accumulated along the margins of a flowing glacier as it moves.

This is a landslide.

This landslide may somewhat resemble the shape of a tongue with the lobes near its base just to the upper right of the centre of the image, but it lacks the flow lines seen in glacial tongues.

South Polar Pits

The south pole of Mars is covered in a large sheet of carbon dioxide ice. Seasonal changes in this polar cap produce pits that make the surface look like Swiss cheese—in fact, scientists even refer to it as "Swiss Cheese terrain!" Repeated imaging of these pits by satellites orbiting Mars has revealed that these pits are expanding over time.

Practice identifying the south polar pits

Mid-latitude crater fill in a southern hemisphere crater
Simple crater in the Libya Montes
Concentric crater fill in a northern mid-latitude crater

These are polygons.

The terrain here is relatively flat except for some small hills, and the roughly square and rectangular shapes of the polygonal terrain. Remember, a polygon is any shape with more than 3 sides.

Yes, these are south polar pits!

The rounded shapes here are pits in the south polar carbon dioxide ice cap of Mars. Can you see the resemblance to Swiss cheese that led scientists to call this "Swiss cheese terrain?"

These are pingos.

This may look like a cluster of pits or craters, but it is actually a group of small pingos. The cone-shaped ones appear to be intact, while the crater-like features that don’t look elevated above the surface may be pingos whose ice cores have melted, causing them to collapse in on themselves.